Crystal structures of chiral mono- and dinuclear salan–Cu(II) complexes and the application to catalytic asymmetric Henry reaction

Two chiral salan–Cu(II) complexes, dinuclear {[(C28H26CuN2O2)2 · 2CH3OH∙CH3COOH] (1)} and mononuclear {[C56H55CuN4O4∙½CH3OH∙CH3COOH∙H2O] (2)} were synthesized and structurally characterized. The complex 1 was obtained with the ratio 1:1.2 of the metal source and the ligand, in which the two copper atoms were bridged by μ–O atoms forming a four-membered binuclear Cu2O2 square–planar geometry. However, using two equivalents of the ligand led to the formation of the complex 2, in which one copper atom coordinated with two salan ligands bound in a five-coordinate square–pyramidal. The asymmetric Henry reaction was studied by these in situ formed complexes. The complex 1 provided moderate to high enantioselectivities (up to 83%) when the reaction was carried out in EtOH at room temperature for 48 h, whereas the complex 2 gave a modest enantioselectivity.